Industrial coating systems employ a conveyor from which workpieces are hung on support racks and are conveyed through several stations, usually including stations for performing the steps of cleaning, rinsing, drying, coating, and baking. In many industrial coating operations, the workpieces travel through an electrostatic coating booth wherein the electrically grounded workpieces are sprayed or coated with electrically charged coating particulates, either liquid or powder. After coating and baking, the coated workpieces are removed from the racks and the racks are reused for another coating cycle. Because of the recycling of the racks, they become coated and encrusted with multiple layers of the coating material. It is important in those instances where electrostatic coating is employed that the support rack be electrically conductive so that workpieces can be maintained in a grounded state. The workpieces are electrically connected to ground through a conductive support rack so that electrically charged particulates are attracted to the workpieces by the electrostatic field. Thus, it is desirable to provide a support rack wherein the electrical contact between individual hooks depending from the rack are maintained in low-resistance electrical contact with the rack so that the workpieces suspended from the hooks will be satisfactorily grounded.
Each coating cycle applies a coating layer to the exposed, workpiece-engaging portion of the support hook, which covers the entire surface of the hook except for the small area of contact with the workpiece. Unless identical workpieces are coated in subsequent cycles, so that the workpiece is in electrical contact with the hook at an identical position, it becomes necessary to clean the support hook to assure the requisite electrical contact between the workpiece and the support hook. Accordingly, it is desirable that the support hooks be detachably engaged with the rack so that coated hooks can be periodically removed and replaced with new or cleaned hooks. Simple and rapid interchangeability of the support hooks is also desirable so that different sizes and shapes of support hooks can be interchanged as desired for the rack to be used for coating a wide variety of workpieces. It is desirable to provide this interchangeability while still maintaining good electrical contact between each support hook and its respective rack during coating operations.
In this connection, it has been found desirable to form a support hook in the shape of a spring to bias a clean surface of the hook against a clean surface of its support rack. An early example of such a cooperative hook and rack construction may be found in U.S. Pat. No. 1,533,805, relating to a rack for electroplating workpieces. A more recent example of such a cooperative hook and rack construction may be found in U.S. Pat. No. 5,531,334, for electrostatically coating workpieces.
Such prior art hook and rack constructions have been found to have certain shortcomings in industrial coating operations, and it is the principal object of the present invention to provide a hook and rack construction having substantially improved performance.